Field of the Invention
The present invention relates to a solid-state image sensor, a method of controlling the same, and a storage medium.
Description of the Related Art
A solid-state image sensing element conventionally used in an image capturing apparatus such as a digital still camera or a digital video camera is required to attain high-speed driving to improve the frame speed or frame rate. As a method of implementing high-speed driving, the drive current is increased. However, increasing the drive current is not preferable because lower power consumption is simultaneously required in consideration of the influence of heat on image quality in these days.
Japanese Patent Laid-Open No. 2004-134867 discloses a technique of reading out a first pixel signal and then adding and reading out a second pixel signal without reset in an arrangement in which two photodiodes (PDs) in proximity share one amplification MOS. The second pixel signal is obtained by subtracting the first pixel signal from the sum signal. According to Japanese Patent Laid-Open No. 2004-134867, since a common reset level signal can be used for the first pixel and the second pixel, the count of reset level signal readout can be reduced by one. The driving method described in Japanese Patent Laid-Open No. 2004-134867 is a very effective means to the above-described problem because the readout time can be shortened without increasing the drive current.
On the other hand, solid-state image sensing elements using a column parallel AD conversion method have become widespread. The solid-state image sensing element using the column parallel AD conversion method can perform AD conversion in it and can therefore perform high-speed signal output.
As one of the column parallel AD conversion methods, a method called a single slope method is widely known. In the single slope method, an electric potential corresponding to a pixel signal is input to one input of a comparator, and a ramp waveform whose electric potential varies with a certain slope as the time elapses is input to the other input as a reference signal. A counter counts a time from the start of comparison up to inversion of the comparator. The count value is output as a digital signal.
In this single slope AD conversion, the counter is one of main power-consuming components because it continues operating from the start of comparison up to inversion of the comparator.
Japanese Patent Laid-Open No. 2010-56707 discloses the following technique. That is, the signal value of a readout row is predicted from the signal value of the same row of a preceding frame or a preceding row of the same frame, which has undergone AD conversion at a preceding timing. The start potential of the reference signal is changed to an electric potential close to the analog pixel signal potential, thereby shortening the operation period of the counter.
Examine driving that combines the technique described in Japanese Patent Laid-Open No. 2004-134867, which can read out a pixel signal at a high speed, and the column parallel AD conversion method capable of high-speed output for faster readout driving. At this time, when AD-converting the sum signal of a first pixel and a second pixel, the signal including the first pixel signal that has already undergone AD conversion is AD-converted again. This leads to a wasteful counter operation and is still problematic from the viewpoint of power consumption.
In the technique of Japanese Patent Laid-Open No. 2010-56707, for example, when predicting the signal value of a readout row from the signal value of the same row of a preceding frame, the signal value of the readout row is predicted from the signal value of pixels that are different in terms of time. For this reason, in case of a high-speed object or the like, the signal value changes at high possibility between the readout row and the same row of the preceding frame. The operation period of the counter may be prolonged because correct prediction is impossible. When predicting the signal value of a readout row from the signal value of a preceding row of the same frame, the signal of the readout row is predicted from pixels that are different in terms of space. Hence, for an object whose luminance changes in the vertical direction, correct prediction may be impossible due to the same reason.